The elimination of acid compounds especially H.sub.2 S and CO.sub.2, contained in a gas, also called deacidification of said gas, is generally carried out by contacting the gas with an absorbent solution that retains the acid compounds by simple physical dissolution and/or by dissolution after formation of a thermally unstable salt or complex by reaction of the acid compounds with a basic compound present in the absorbent solution.
In practice, the gas to be treated, which contains the acid compounds to be removed, is brought into contact, in a zone called an absorption zone, with the absorbent solution under pressure and temperature conditions such that the absorbent solution almost entirely fixes the acid compounds, the purified gas exiting at the head of the absorption zone. The absorbent solution, charged with the acid compounds, is recovered at the bottom of the absorption zone. The charged absorbent is subjected to a regeneration treatment which separates the acid compounds from the absorbent and restores its absorbing power with respect to the acid compounds. The regeneration is carried out by introducing the charged absorbent solution from the absorption zone into the upper portion of a regeneration zone. The absorbent solution to be regenerated is maintained in this zone under temperature and pressure conditions adequate for allowing a release and/or stripping of the absorbed acid compounds. Regeneration is effected by maintaining the absorbent solution boiling under pressure in the regeneration zone. The heat needed for boiling the absorbent solution is supplied by indirect heat exchange between a portion of the solution to be regenerated, which is in the lower half of the regeneration zone, and a hot fluid at an adequate temperature, generally of saturated steam. In the course of the regeneration, the acid compounds contained in the absorbent solution to be regenerated are released and stripped by the vapors of the absorbent solution and are removed at the head of the regeneration zone. At the bottom of the regeneration zone, the regenerated absorbent solution is recovered and recycled to the absorption zone. The regenerated absorbent can be cooled by indirect heat exchange with the solution to be regenerated and recovered from the absorption zone prior to introduction into the regeneration zone.
When the gas to be deacidified contains mercaptans in addition to H.sub.2 S and/or CO.sub.2, the mercaptans are not substantially removed from the gas in the course of washing the gas with the absorbent solution. In fact, if the conditions are suitable for absorption of the mercaptans at the head of the absorption zone, the absorbent solution, when descending into the zone, is, on the one hand, charged with H.sub.2 S and CO.sub.2 by physical or chemical dissolution and on the other hand heated due to the exothermic nature of the dissolution reactions. The two phenomena cause the mercaptans that have been physically absorbed at the head of the absorption zone to be desorbed and escape for the most part into the treated gas emerging at the head of the absorption zone.
The object of the invention is to provide a process for deacidification of a gas containing H.sub.2 S and/or CO.sub.2 and also mercaptans by washing with a regeneratable absorbent solution which not only removes the H.sub.2 S and CO.sub.2 acid compounds contained in the gas but also a sufficient amount of mercaptans to provide a treated gas which can meet the specification for treated gas.